1. Field of the Invention
The present invention relates to a rotor for a permanent-magnet motor having a vibration isolation construction for use in air conditioners, hot-water supply units or the like.
2. Description of the Related Art
In recent years, permanent-magnet motors have become widely used in fan motors because of good controllability. A conventional rotor of a permanent-magnet motor will be described below with reference to FIGS. 6, 7A and 7B.
FIG. 6 illustrates a DC brushless motor, one type of a permanent-magnet motor. As shown in FIG. 6, the permanent-magnet motor comprises a stator core 9 and a rotor core 6. A permanent magnet 5 is fixed around the outer periphery of the rotor core 6, a shaft 7 is fixed around the central portion thereof, and the shaft 7 is rotatably assembled onto a bearing 8 of a rotor frame in which the rotor core 6 is disposed. A fan 10 is fixed to the end portion of the shaft 7 by means of a nut 13.
With the above-described construction, since the rotor core 6 has insufficient vibration attenuation properties, vibrations along the path of rotation generated in a rotor due to rotary magnetic fields of the stator are transmitted to the fan 10 via the shaft 7, causing the fan 10 to vibrate and make an unpleasant noise.
As a measure for preventing such unpleasant noise, there is commonly used means whereby rubber 11 is assembled on both sides in a direction perpendicular to the thrust of the shaft 7 by a sandwich structure by making use of a fan receiving plate 14 for reducing noise generated due to resonance of the fan 10 when the fan 10 as shown in FIG. 8A is fixed to the rotary shaft 7, or means whereby rubber 12 is inserted between the shaft 7 and the fan 10 by means of bosses 15 and 16. Rubber is commonly used to prevent vibration. As a means for reducing generated noise, from an electrical point of view, skew magnetization is performed.
However, in the prior art, since use of rubber increases the number of assembly steps or makes the shape of the fan complex, the cost is high, and resistance of rubber to heat is problematical. Also, when skew magnetization is performed, efficiency, output and the like decrease due to loss of magnetic flux.